1. Field of the Invention:
This invention relates to the construction of a pipe for plants and the techniques for manufacturing the pipe.
2. Description of the Prior Art:
As generally known, an oil well pipe and a pipe used for the piping in a chemical plant and a nuclear power plant require to have not only the pressure resistance and thermal resistance but also the corrosion resistance.
A high alloy pipe, such as a stainless steel pipe or a nickel (Ni) based alloy pipe has been used as a corrosion resisting steel pipe which meets these conditions. However, such alloys are expensive. With a view a reducing the material cost, a means for providing a double pipe has been proposed, which consists of an outer heat resisting and pressure resisting carbon steel pipe member, and an inner pipe member composed of a corrosion resisting metal, such as stainless steel and attached to the inner surface of the outer pipe member metallurgically or mechanically to furnish the outer pipe member with the corrosion resistance.
For example, as shown in FIGS. 1 and 2, the inner austenite steel pipes 2, are inserted in the outer carbon steel pipes 1, to be expanded by a liquid pressure and thereby fit the inner pipes 2, closely in the outer pipes 1. The ends of the inner pipes 2, are then welded 3, to the inner surfaces of the outer pipes 1, to seal the same as shown in FIGS. 3 and 4.
The ends of these unit end pipes are joined together by butt welding 4 the outer pipes 1, as shown in FIG. 5, and then build-up welding 5 the ends of the inner pipes 2, with a filler metal, which consists of austenite stainless steel, as shown in FIG. 6.
It is known well that, in order to join an inner corrosion resisting pipe to an outer pipe in the above-mentioned manner for the purpose of obtaining a corrosion resisting steel pipe having a high strength, it is better to use a mechanical lining method than to use a metallurgical method, i.e., a clad-forming method.
It is technically difficult, in general, to join an inner pipe to an outer pipe by using, for example, titanium (Ti) having a high corrosion resistance, or a titanium (Ti) metal, such as a titanium (Ti) alloy (these metals will hereinafter be referred to as generally "Ti metals").
It has recently become technically possible to metallurgically combine a Ti metal with carbon steel by a means such as explosive cladding or diffusion bonding.
In general, as the lengths of pipes increase, the number of joint portions thereof decrease, and the stability thereof with respect to the strength increases. Moreover, the pipes of larger lengths enable a piping operation to be simplified. Since the lengths of pipes are limited by a pipe-manufacturing apparatus used in a factory and a transportation means used to carry pipes from a factory to a place of piping, the unit lengths of pipes are standarized.
There is, of course, a means, which is used in a special case, for combining pipes together and then plastically processing the resultant pipes. However, there is a difference is general between the processability of a carbon steel pipe and that of a corrosion resisting Ti metal, so that it is said to be difficult to plastically process a corrosion resisting steel pipe of a large length.
While the corrosion resisting steel pipes of a set unit length manufactured in a factory are transported to a building site so as to be arranged therein, they are stored in the open air temporarily in some cases. In such cases, there is the possibility that the corrosion progresses with the lapse of time on the inner side of the joint surfaces of the end portions of the outer pipe and the inner corrosion resisting pipe joined to the inner surface of the outer pipe. Therefore, it is necessary in general that the pipe ends be seal-welded.
However, even when the carbon steel constituting the outer pipe and the corrosion resisting Ti metal constituting the inner pipe on the inner surface of the outer pipe are combined together by using an intermediate material, the attainment of a sufficiently high bond strength of the joint portion of the pipes cannot be expected.
Although the corrosion resistance of the corrosion-resisting Ti metal applied to the inner surface of an outer steel pipe in the above-mentioned manner is high, it is metallurgically inconvenient to seal-weld this metal at an end of the steel pipe.
Also, when steel pipes made to a unit length are joined together, it is difficult to seal them completely in a non-corrosion-progressing state due to the above-mentioned inconveniences.